1. Field of the Invention
The present invention relates to a method of fabricating a compound semiconductor device employed for an LED (light emitting diode) or the like and an apparatus for fabricating a compound semiconductor device, and more particularly, it relates to a method of fabricating a semiconductor device for a ZnSe-based LED employed for the backlight of a liquid display unit or the like and an apparatus for fabricating a compound semiconductor device.
2. Description of the Prior Art
In order to drive a compound semiconductor device such as an LED, it is necessary to form an electrode on a compound semiconductor. For example, an electrode of ohmic contact must be formed on the back surface of a ZnSe substrate, in order to produce a ZnSe-based LED chip. However, such an electrode of ohmic contact cannot be readily formed on the ZnSe substrate for the following reasons:
(a) The upper bound of the carrier concentration of the ZnSe substrate is limited to the latter half of the 1017 mark. No ZnSe substrate having a carrier concentration exceeding the latter half of the 1017 mark has heretofore been fabricated. In particular, the carrier concentration of a p-type ZnSe substrate cannot even reach the 1017 mark.
(b) Oxides are readily formed on the surface of the substrate.
(c) The treatment temperature for forming the electrode must be not more than 250xc2x0 C., in order to protect an active layer or a cladding layer of an emission part.
In general, In which is a low melting point metal is known as an electrode metal implementing ohmic contact. An electrode of ohmic contact can be formed also on the aforementioned ZnSe substrate by fusing In.
In a ZnSe-based LED prepared by fusing In, however, various inconveniences result from the low melting point of In in solder reflow or transfer molding. When the ZnSe-based LED is heated to 200xc2x0 C. to 250xc2x0 C., for example, ball-up results from the low melting point of In of about 157xc2x0 C., and hence a flat interface cannot be obtained. Therefore, uniform ohmic contact cannot be attained despite implementation of partial ohmic contact. When a flat electrode having ohmic contact is not formed, an unnecessarily high voltage must be applied to the overall LED, which in turn requires a large number of batteries and cannot be readily applied to the backlight for a liquid crystal display screen of a portable telephone or the like. Thus, strongly awaited is development of an electrode, not prepared from In, capable of attaining thermally and mechanically stable ohmic contact with a compound semiconductor.
An object of the present invention is to provide a method of fabricating a compound semiconductor device having an electrode attaining stable ohmic contact with a compound semiconductor without employing a low melting point metal such as In and an apparatus for fabricating a compound semiconductor device.
The method of fabricating a compound semiconductor device according to the present invention comprises a substrate cleaning step including a first cleaning step of heating a compound semiconductor substrate containing a first conductivity type impurity in a temperature range of not more than 250xc2x0 C. for etching its surface with hydrogen chloride and a second cleaning step of performing a radical hydrotreatment on the compound semiconductor substrate etched with hydrogen chloride after the first cleaning step.
When etching the surface of the compound semiconductor substrate with hydrogen chloride, an oxide film resulting from atmospheric exposure can be removed. In this hydrogen chloride etching, the temperature of the compound semiconductor substrate must be not more than 250xc2x0 C., in order to prevent damage of an active layer and a cladding layer already formed on the opposite surface of the compound semiconductor substrate. When performing the hydrogen chloride treatment at a temperature of not more than 250xc2x0 C., the surface of the compound semiconductor substrate adsorbs Cl although the oxide film or a carbide can be removed from this surface. Therefore, the radical hydrotreatment is performed for removing the adsorbed Cl. Cl can be removed by the radical hydrotreatment. Radical hydrogen has high reaction activity, and hence a sufficiently high radical hydrogenation reaction rate can be ensured also when setting the temperature of the compound semiconductor substrate to not more than 250xc2x0 C. A clean surface of a compound semiconductor can be obtained also at a temperature of not more than 250xc2x0 C. due to the hydrogen chloride treatment and the radical hydrotreatment, so that an epitaxial compound semiconductor film can be formed without forming interfacial levels. The compound semiconductor substrate includes not only the bare substrate in the initial stage of the treatments but also the compound semiconductor substrate formed with a thin film such as an optical active layer in the process of the treatments. The present invention is mainly directed to a case of forming an ohmic electrode layer on the back surface of a compound semiconductor substrate already formed with an optical active layer or the like on its surface without damaging the optical active layer or the like.
The method of fabricating a compound semiconductor device according to the present invention can further comprise a compound semiconductor film forming step of epitaxially growing a compound semiconductor film containing the first conductivity type impurity in a higher concentration than the compound semiconductor substrate on the compound semiconductor substrate after the cleaning step and a conductive electrode film forming step of forming a conductive electrode film on the compound semiconductor film.
When directly forming a conductive electrode layer of a material other than In on the compound semiconductor substrate, ohmic contact cannot be attained if the compound semiconductor substrate has a low carrier concentration. Therefore, the compound semiconductor film containing a conductive impurity in a higher concentration than the compound semiconductor substrate is epitaxially grown for forming the conductive electrode film on the epitaxial film having a high carrier concentration and ensuring ohmic contact. If merely performing cleaning by general etching after exposing the surface of the compound semiconductor substrate to the atmosphere, a desired compound semiconductor film cannot be obtained due to a large quantity of impurities remaining on the surface. Therefore, when performing film formation at a low temperature of not more than 250xc2x0 C., for example, an epitaxial film having a high carrier concentration cannot be obtained with a small number of interfacial levels on the surface of the compound semiconductor substrate.
If the interface between the compound semiconductor film and the conductive electrode film is not clean, large interfacial resistance is formed to cause such large potential difference on the interface that ohmic contact is not implemented, or voltage applied to the overall LED cannot be reduced even if ohmic contact is implemented. According to the aforementioned structure of the present invention, a highly clean surface can be obtained on the back side of the compound semiconductor substrate without damaging an optical active layer or the like already formed on its surface, for readily forming an electrode of ohmic contact thereon.
An active layer and a cladding layer serving as emission parts are formed on another surface of an n-type ZnSe substrate. The active layer and the cladding layer stably emit blue light, while the ZnSe substrate receiving this blue light emits yellow light. Therefore, highly stable white light can be obtained through the low-priced compound semiconductor device. According to the present invention, the electrode of ohmic contact is provided on the back surface of the aforementioned n-type ZnSe substrate for reducing a contact potential, thereby enabling reduction of the number of batteries necessary for the backlight of a liquid crystal display unit for a portable terminal or the like.
The aforementioned method of fabricating a compound semiconductor device according to the present invention can carry out treatments in the compound semiconductor film forming step and the conductive electrode film forming step within such a temperature range that the temperature of the compound semiconductor substrate is not more than 250xc2x0 C.
Thus, the electrode layer of ohmic contact can be readily formed on the back surface of the compound semiconductor substrate without damaging an active layer or the like already formed on the surface of the compound semiconductor substrate.
In the aforementioned method of fabricating a compound semiconductor device according to the present invention, treatments in the substrate cleaning step, the compound semiconductor film forming step and the conductive electrode film forming step are preferably continuously carried out without exposing the compound semiconductor substrate to the atmosphere, for example.
If both of the interface between the compound semiconductor substrate and the compound semiconductor film and that between the compound semiconductor film and the conductive electrode film are not cleaned, large interfacial resistance is formed to cause large potential difference on the interfaces. If the interface between the compound semiconductor substrate and the compound semiconductor film is exposed to the atmosphere, the effects of the hydrogen chloride treatment and the radical hydrotreatment are reduced by half. The interface between the compound semiconductor film and the conductive electrode film tends to be exposed to the atmosphere. When exposed to the atmosphere, the interface is contaminated with oxides or carbon and hence ohmic contact is hard to implement or voltage applied to the overall LED cannot be reduced even if ohmic contact is implemented. When the compound semiconductor device is used as the backlight of a liquid crystal display unit for a portable terminal, therefore, the number of necessary batteries cannot be reduced. Ohmic contact of low resistance can be implemented by continuously forming the compound semiconductor film and the conductive electrode film on the clean surface of the substrate and the compound semiconductor film respectively without atmospheric exposure, as described above.
In the aforementioned method of fabricating a compound semiconductor device according to the present invention, treatments in the substrate cleaning step, the compound semiconductor film forming step and the conductive electrode film forming step are preferably carried out in respective treatment chambers of a treatment apparatus having a plurality of treatment chambers coupled with each other by an ultrahigh vacuum transfer path respectively without exposing the compound semiconductor substrate to the atmosphere between the treatments, for example.
The compound semiconductor substrate can be heated to a temperature of not more than 250xc2x0 C. not damaging the optical active layer or the like without atmospheric exposure, for obtaining a clean surface in an assembly-line manner and forming the electrode film of ohmic contact. Thus, the electrode film of ohmic contact can be extremely readily formed on the back surface of the compound semiconductor substrate, for improving the fabrication yield of an LED emitting white light, for example, and reducing the fabrication cost.
In the aforementioned method of fabricating a compound semiconductor device according to the present invention, the first cleaning step can be carried out under conditions of (a) gas components 5 to 20 volume % of HCl and a rest of He gas, and (b) gas pressure of 1xc3x9710xe2x88x926 Torr to 1xc3x9710xe2x88x924 Torr, for example.
The surface of the compound semiconductor substrate is positively etched due to the aforementioned hydrogen chloride cleaning, so that oxides are removed. Thus, one condition for epitaxially growing a high-concentration carrier film is satisfied. In relation to the gas composition with the rest of He gas, oxides are insufficiently removed if the content of HCl is less than 5 volume %, while the etched surface is not flattened if the content exceeds 20 volume %. Further, the etching rate is so small that etching is practically impossible if the total gas pressure is less than 1xc3x9710xe2x88x926 Torr in the aforementioned gas composition, while etching is ununiformly performed to deteriorate surface flatness if the gas pressure exceeds 1xc3x9710xe2x88x924 Torr. If the substrate temperature exceeds 250xc2x0 C., further, the performance of a portion formed as the active layer etc. of the LED is damaged.
In the aforementioned method of fabricating a compound semiconductor device according to the present invention, hydrogen radicalization in the second cleaning step can be carried out under conditions of (a) hydrogen pressure of 5xc3x9710xe2x88x927 Torr to 5xc3x9710xe2x88x924 Torr, and (b) radicalization power of 50 to 300 W, for example.
Adsorbed Cl can be removed for obtaining a clean surface by the radical hydrotreatment carried out at a temperature of not more than 250xc2x0 C. under the aforementioned conditions.
In the aforementioned method of fabricating a compound semiconductor device according to the present invention, the compound semiconductor substrate can be an n-type ZnSe substrate containing an n-type impurity of at least 1xc3x971017/cm3, for example, the compound semiconductor film can be an n+-type ZnSe film containing an n-type impurity of at least 1xc3x971019/cm3, and the conductive electrode film can include a Ti film in contact with the n+-type ZnSe film and a protective film protecting the Ti film.
An active layer and a cladding layer serving as emission parts are formed on the other surface of the n-type ZnSe substrate. The active layer etc. stably emit blue light, while the ZnSe substrate receiving this blue light emits yellow light. Therefore, highly stable white light can be obtained through the low-priced compound semiconductor device. The electrode of ohmic contact according to the present invention is provided on the back surface of the aforementioned n-type ZnSe substrate for reducing a contact potential, thereby enabling reduction of the number of batteries necessary for the backlight of a liquid crystal display unit for a portable terminal or the like. Electric resistance can be reduced by setting the n-type impurity concentration in the n-type ZnSe substrate to at least 1xc3x971017/cm3, while ohmic contact can be implemented by setting the n-type impurity concentration of the n+-type ZnSe film to at least 1xc3x971019/cm3. If the n-type impurity concentration of the n+-type ZnSe film is less than 1xc3x971019/cm3, ohmic contact cannot be attained between this film and a conductive layer such as the Ti film but interfacial resistance is disadvantageously increased. If the n-type impurity concentration in the n-type ZnSe substrate is less than 1xc3x971017/cm3, electric resistance is disadvantageously increased. Such increase of the resistance increases the number of necessary batteries, to increase the weight of and the cost for the compound semiconductor device.
Ti readily implements ohmic contact with respect to the compound semiconductor film containing the aforementioned first conductivity type impurity, such as an n-type impurity, for example, in a high concentration. Therefore, the Ti film is formed to implement ohmic contact, and a chemically stable Au film, for example, is formed on the active Ti film.
The apparatus for fabricating a compound semiconductor device according to the present invention comprises a gas treatment apparatus performing gas etching on a compound semiconductor substrate, a radical treatment apparatus performing a radical treatment on the compound semiconductor substrate, a film formation treatment apparatus forming a compound semiconductor film on the compound semiconductor substrate and a conductive film forming apparatus forming a conductive film on the compound semiconductor substrate, and further comprises an ultrahigh vacuum transfer path coupled with the respective ones of the gas treatment apparatus, the radical treatment apparatus, the film formation treatment apparatus and the conductive film forming apparatus to be capable of transferring the compound semiconductor substrate.
It is possible to continuously perform a hydrogen chloride treatment and a radical hydrotreatment in an ultrahigh vacuum, reduce interfacial resistance and form an epitaxial compound semiconductor film by employing the treatment apparatus having the aforementioned structure. The ultrahigh vacuum is a vacuum of less than 10xe2x88x928 Torr. The compound semiconductor device according to the present invention can be fabricated only by the fabrication apparatus having the aforementioned structure.
In the aforementioned apparatus for fabricating a compound semiconductor device according to the present invention, the gas treatment apparatus can include a hydrogen chloride treatment apparatus etching the compound semiconductor substrate with hydrogen chloride, for example.
Oxygen and carbon adhering to the surface of the compound semiconductor substrate can be removed by positively etching the compound semiconductor substrate with the hydrogen chloride treatment apparatus connected by the ultrahigh vacuum transfer path, for moving the compound semiconductor substrate to a next step in the ultrahigh vacuum.
In the aforementioned apparatus for fabricating a compound semiconductor device according to the present invention, the radical treatment apparatus can include a radical hydrotreatment apparatus performing a radical hydrotreatment on the compound semiconductor substrate, for example.
When performing the hydrogen chloride treatment at a temperature of not more than 250xc2x0 C., Cl is absorbed on the compound semiconductor substrate to form interfacial levels when the compound semiconductor film is formed on the compound semiconductor substrate. Therefore, the radical hydrotreatment is carried out after the hydrogen chloride treatment while moving the compound semiconductor in the ultrahigh vacuum without atmospheric exposure, thereby removing Cl so that a clean surface appears and an epitaxial film can be formed without interfacial levels.
The aforementioned apparatus for fabricating a compound semiconductor device according to the present invention can have an MBE (molecular beam epitaxial) apparatus serving both as the radical hydrotreatment apparatus and the film formation treatment apparatus, for example, and the MBE apparatus can include a Zn cell, an Se cell, a ZnCl2 cell, a hydrogen gas supply source and a radicalization gun for performing a radical hydrotreatment on the compound semiconductor substrate with the hydrogen gas supply source and the radicalization gun and forming an n-type ZnSe film on the compound semiconductor substrate with the Zn cell, the Se cell and the ZnCl2 cell.
A highly clean surface of the compound semiconductor substrate can be obtained due to the aforementioned structure, so that an epitaxial n+-type ZnSe film having a high carrier concentration can be formed thereon. Thus, it is possible to implement ohmic contact with respect to a substrate whose carrier concentration can be increased to merely the latter half of the 1017 mark.
In the aforementioned apparatus for fabricating a compound semiconductor device according to the present invention, the conductive film forming apparatus can form at least one of a Ti film and an Au film on the compound semiconductor substrate.
A back electrode implementing ohmic contact with respect to the compound semiconductor device can be formed due to the aforementioned structure.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.